1. Field of the Invention
The present invention relates to a method and apparatus for the scheduling of wireless access in a local area network (LAN) environment, and more particularly, to a method and apparatus for providing prioritized access in a carrier sense multiple access with collision avoidance (CSMA/CA) wireless LAN in which each terminal component of the LAN runs an independent priority protocol to prevent access collisions all without the necessity of a centralized infrastructure.
2. Description of the Art
As the deployment and use of indoor wireless data networks continues to grow, engineers are faced with a growing demand to run a mix of different devices/applications on these networks. The use of more devices/applications on a given network, however, creates a problem in that each of these different devices/applications is in conflict with the others over priority of transmission.
One proposed solution uses a centralized scheduler at the medium access control (MAC) layer to differentiate between classes of service offered to the devices/applications over the network. This idea evolved from the solution found in wire-line networks where different classes of service were provided along with signaling protocols to reserve network resources on a per-flow basis. It was extended to wireless LANs by the concept of reserving time slots on the shared wireless medium. To provide different classes of service, the centralized scheduler divides available bandwidth into two time periods. In the first period, stations contend for medium access using a random back-off to avoid collisions. In the second period, the centralized scheduler (typically located at the base station) offers contention-free access to selected terminals for a given number of time slots. Much of this work was in the context of a wireless asynchronous transfer mode (ATM), where slots can be reserved in response to ATM signaling messages associated with flows to/from the wireless stations. These types of systems use MAC-layer mechanisms like polling or token passing.
This solution, however, suffers from a number of problems. Having a MAC layer which provides both scheduled and contention-free periods is complex, increases access latency and decreases bandwidth utilization. Polling and token passing introduce overheads which can be significant relevant to the limited bandwidth capacity. This is due to extra packets or header fields for protocol information and latencies for exchanging state information, timeouts for lost tokens and stations out of range, etc. Thus, using a centralized scheduler prohibits the formation of so-called ad-hoc networks and has the using problems of scaling and reliability associated with centralized systems.
An alternative solution for wireless LANs, on the other hand, is the fundamental access method described in published IEEE 802.11 draft standard entitled xe2x80x9cWireless LAN Medium Access Control and Physical Layer specificationsxe2x80x9d, 1997, and which is hereby incorporated by reference. The standard defines the specifications for wireless access for fixed, portable and moving stations on a LAN and gives two operative methods for doing so. The operative methods are based on what is referred to as carrier sense multiple access with collision avoidance (CSMA/CA) and operate by performing carrier sensing based on a channel assessment function provided by the physical layer (i.e., the interface of the devices/applications to be used/run over the network). Prior to transmission a station senses the medium. If the medium is not determined to be busy, the station monitors the medium for a specified amount of time. If the medium remains continuously idle during this time, then the station invokes a collision avoidance protocol. In the collision avoidance protocol, the station defers transmission of its packet for a randomly selected back-off duration. If no other station transmits during this back-off duration, then the station starts transmitting its packet. Otherwise, the station once again waits for the medium to become idle.
Unfortunately, however, this solution method is essentially only a xe2x80x9cbest-effortxe2x80x9d service and thus does not fully take into account the delay sensitivity, bandwidth demands and error tolerance of traffic awaiting transmission at the individual devices.
Accordingly, the present invention allows for a method and apparatus having a decentralized prioritized scheduling of wireless access in which individual devices/applications can determine their own transmission priority based upon their data packet""s delay sensitivity, bandwidth demand and error tolerance.
Each device/application in the CSMA/CA medium determines its own priority by running a modified distributed coordination function (DCF) in which a prioritized collision avoidance protocol is invoked. The prioritized collision avoidance protocol maintains a back-off timer for each flow, rather than on just a per-station basis, and, for each flow (and thus each back-off timer associated with that flow) associates a priority parameter. Each device/application in the CSMA/CA wireless LAN is thus able to select the most urgent of its"" flows such that the highest priority flow back-offs for the shortest period of time. The priority parameter itself can be set, or adapted to the flow""s priority, by invoking a back-off algorithm.